1. Field of the Invention
The present invention relates to a method for combusting pulverized coal, and more particularly to a method for combusting of pulverized coal including the steps of separating pulverized coal mixture gas ejected from a vertical type coal grinder containing a rotary type classifier therein into thick mixture gas and thin mixture gas by means of a distributor, and injecting these thick and thin mixture gases respectively through separate burner injection ports into a common furnace to make them burn.
2. Description of the Prior Art
One example of the method for combusting pulverized coal in the prior art is shown in a system diagram in FIG. 8. In this figure, reference numeral 01 designates a vertical type coal grinder containing a stationary type classifier therein, numeral 2 designates a pulverized coal pipe, numeral 3 designates a distributor, numeral 4 designates a thick mixture gas feed pipe, numeral 5 designates a thin mixture gas feed pipe, numeral 6 designates a thick mixture gas burner, numeral 7 designates a thin mixture gas feed pipe, numeral 8 designates a boiler furnace.
Pulverized coal mixture gas consisting of coal pulverized finely by the vertical type coal grinder 01 and primary air for combustion is, after having been ejected from the coal grinder and introduced into the pulverized coal pipe 2, separated into thick mixture gas and thin mixture gas by the distributor 3. The thick mixture gas passes through the thick mixture gas feed pipe 4 and is injected from the thick mixture gas burner 6 into the boiler furnace 8 to burn. On the other hand, the thin mixture gas passes through the thin mixture gas feed pipe 5 and is injected from the thin mixture gas burner 7 into the boiler furnace 8 to burn. In such a pulverized coal combustion method in the prior art, by separating pulverized coal mixture gas into thick mixture gas and thin mixture gas and making them burn separately, a suppression of the production of nitrogen oxides (NO.sub.x) in the course of the combustion reaction is effected, and, therefore, in recent low NO.sub.x combustion apparatuses, such a method is most frequently employed.
One example of a vertical type coal grinder 01 containing a stationary type classifier is shown in a longitudinal cross-sectional view of FIG. 9. In this figure, material to be ground such as lumped powder coal or the like charged through a feed pipe 10 is subject to a load, on a rotary table 20 by a grinding roller 30 and is thus ground into pulverized coal, and is spattered towards the outer circumference of the same rotary table 20. On the other hand, hot air is issued from a hot air inlet port 40 at the lower portion of the coal grinder 01 through a blow-up portion 50 into a mill. The above-mentioned pulverized coal spattered towards the outer circumference of the rotary table 20 is blown to the upper portion of the coal grinder 01 by this hot air, that is, by this carrier gas, passes through stationary vanes 80 and is fed into a stationary type classifier 60, where it is separated into fine powder and coarse powder. Then the fine powder is taken out through a pulverized coal pipe 110, while the coarse powder falls along the inner circumferential wall of the stationary type classifier 60 onto the rotary table 20 and is ground again.
In the above-described pulverized coal combustion method in the prior art, in order to reduce the amount of unburnt material in ash in the boiler constituting loss, it is desirable to make the degree of pulverized coal to be burnt as fine as possible. However, if the degree of pulverization is made excessively high, a degradation of the capability of the grinder and an increase of power consumption would become remarkable. And, moreover, problems such as the generation of vibrations would arise. Therefore, in the pulverized coal combustion method making use of a vertical type coal grinder containing a stationary type classifier therein, it is a common practice to operate the machine with a degree of pulverization of 200 mesh pass 80% or less. A general characteristic of a vertical type coal grinder containing a stationary type classifier therein is shown in FIG. 10. As shown in this figure, in the case where pulverization has been effected by the above-mentioned grinder up to a degree of pulverization of about 200 mesh pass 80%, in the pulverized coal are contained coarse particles of 100 mesh or larger by about 2.4%, representing an inevitable phenomenon which is characteristic of a stationary type classifier.
Now, the mixture gas of pulverized coal ground in the above-described manner is separated into thick mixture gas and thin mixture gas by means of a distributor. However, since the distributor utilizes a classifying effect based on inertial forces, it is inevitable that most of the above-mentioned coarse particles of 100 mesh or larger tend to flow to the side of thick mixture gas. One example of the configuration of the above-described distributor is shown in FIG. 11. In this figure, pulverized coal mixture gas introduced into the distributor through a pulverized coal mixture gas inlet 3a is separated into thick mixture gas and thin mixture gas due to inertial forces, and the mixture gases are ejected, respectively, through a thick mixture gas outlet 3b and a thin mixture gas outlet 3c. In the above-mentioned distributor, while coarse particles of 100 mesh or larger are contained by 2.5% in the pulverized coal at the inlet, 95% or more of such particles are ejected through the thick mixture gas outlet 3b.
The thick mixture gas burner suppresses production of nitrogen oxides by burning pulverized coal within a low oxygen content atmosphere containing air less than a theoretical combustion air amount. However, in the above-described thick mixture gas there is contained a large amount of coarse particles of 100 mesh or larger. Because these coarse particles cannot fully burn out within the low oxygen content atmosphere, most of such particles remain as an unburnt material in ash. Therefore, an unburnt ash component of the mixture gas is high, resulting in a problem of high loss of efficiency in the boiler. A general relation between a degree of pulverization and an unburnt ash content is shown in FIG. 12.
On the other hand, from a view point of effective utilization of coal, the necessity for suppressing an unburnt ash content to less than a regulated value would often arise. And, in such cases since operations for increasing a surplus air proportion are necessitated, there was a problem in that the production of nitrogen oxides could not be effectively suppressed. Relations between a surplus air proportion and an NO.sub.x content as well as an unburnt ash content in the above-described combustion method in the prior art are shown in FIG. 13.
Furthermore, the dashed line curve of FIG. 7 represents a relation between an unburnt ash content and an NO.sub.x content in the pulverized coal combustion method in the prior art. Among these contents, if one is reduced, then the other tends to increase, and so, in order to reduce both the unburnt ash content and the NO.sub.x content, a novel technique is necessary.
In addition, relations between a concentration ratio of the thick mixture gas to the thin mixture gas and an NO.sub.x content as well as an unburnt ash content are established as shown in FIG. 14. If the concentration ratio is increased, the NO.sub.x content is lowered but the unburnt content is increased. Accordingly, in order to maintain both the NO.sub.x content and the unburnt ash content at proper values, it would be necessary to arbitrarily and automatically control the aforementioned concentration ratio according to variations of a boiler load and the kind of coal employed. However, in the pulverized coal combustion method in the prior art, such control was impossible.